Laser microaperture measurement instrument

ABSTRACT

A method and apparatus for determining the diameter of roughly circular apertures by employing a laser to pass a beam of coherent light through the aperture whose diameter is to be measured so that a Fraunhofer diffraction pattern is produced. The Fraunhofer pattern is split by appropriate means and the two identical patterns thus produced each focused upon appropriate photodetectors. For a first reference photodetector the entire Fraunhofer pattern is allowed to be incident upon the reference photocell while for the second detector only part of the inner portion of the bright central area of the pattern known as the Airy Disc is detected. The ratio of the voltages produced by the two photocells is then a function of the diameter of the aperture through which the beam has passed and by appropriate circuitry an output signal related to that diameter can be produced.

United States Patent [151 3,643,101 Shipp et al. 1 Feb. 15, 1972 [54]LASER MICROAPERTURE ()"I'HHR PUHLK'A'IIONS MEASUREMENT INSTRUMENT GermanPrimed Application 1,229,739, Dec. l, I966. Biihm 721 lnventors: John 1.Shipp; Nathan E. Welch; Thomas M PP Q 356/ D. Broadbent, all ofTullahoma, Tenn. Primary Examiner.lames W, Lawrence Assistant ExaminerD.C. Nelms [73] Assignee. aSylsgllrllts & Electrontcs, lnc., Tul- Attorneycushman, Darby & Cushman 22] Filed: Jan. 7, 1969 [571 ABSTRACT [2]] ApplN 789 458 A method and apparatus for determining the diameter of Iroughly circular apertures by employing a laser to pass a beam ofcoherent light through the aperture whose diameter is to be [52] US. Cl..250/2l6, 350/162, 356/156 measured so that a Fraunhofer diffractionpattern is produced. [51] Int. Cl. ..G02b 5/18 Th Fraunhofer pattern issplit by appropriate means and the [58] Field of Search ..350/ 162;250/216; 356/156, two identical attems thus roduced each focused uponap- P P 35 9, 110 propriate photodetectors. For a first referencephotodetector the entire Fraunhofer pattern is allowed to be incidentupon I References Cited the reference photocell while for the seconddetector only part of the inner portion of the bright central area ofthe pat- UNITED STATES PATENTS tern known as the Airy Disc is detected.The ratio of the volt- 3,247,467 8/1962 Geusic et a1. ..331/94.5 agesProduced y the two photocells is then a function of the 3,518,007 6/1970Ito ..356/1 13 x diameter of the aperture through which the beam hasPassed 3,435,239 3/1969 Stilberg ..250/220 SDS and by PP P circuitry anoutput Signal related to that g diameter can be produced.

14 Claims, 2 Drawing Figures 34 r lessee 25 k I .92 f 5a 14 7% dflfpflfVd/Tfi'' (286w 77E) flaw/42m LASER MICROAPERTUIRE MEASUREMENT INSTRUMENTBRIEF DESCRIPTION OF THE PRIOR ART AND SUMMARY OF THE INVENTION Theinvention relates to a method and apparatus for determining thedimensions of holes or apertures.

Many different structures and devices used in a variety of ways, for avariety of diverse purposes employ apertures or holes located in variousplaces on and within such devices or structures. Frequently, theseapertures or holes must be produced with dimensions within narrowtolerances, in great numbers, and, often only a few microns in diameter.Manually monitoring and checking the dimensions and particularly thediameters of such apertures or holes, even when they are of relativelylarge size and reasonable number, is both a tedious and difficult chorewhich is both costly and time consuming. Moreover, when the aperturesare extremely minute, and when great accuracy is required, the jobsurpasses the capabilities of a human inspector and has been, in thepast, accomplished by or with the aid of complex and sophisticateddevices. Not only are such devices extremely complicated and expensive,but, even with their use, it is often difficult to obtain the requisiteaccuracy and consistently to measure the dimensions of such aperturesfor a number of years without frequent and substantial errors.

One particular art in which the need to measure apertures quickly,easily and accurately is particularly acute in the manufacture ofdevices which produce synthetic fibers, fiherglas and other similarmaterials by rapidly spinning a bowl containing the molten material sothat fibers are extruded through numerous extremely small holes in thebowl which ordinarily range in diameter from l to 90 microns. Since eachof these howls, or spinnerettes as they are sometimes referred to.frequently have from 40 to 200 or more roughly circular holes. and thediameter of each hole normally takes about 3 minutes to manuallymeasure, the expense of checking the hole sizes alone is a substantialportion of the cost of the device.

The present invention relates to an aperture-measuring system which canquickly, easily and accurately measure the diameter of such holes, andwhich, in fact, can measure all of the holes in a spinnerette, asdescribed above, in about 30 seconds simply by rotating the spinnerettehead so that all the holes are sequentially presented to a stationarylaser beam, or alternatively moving the laser beam so that it passesthrough each ofthe holes in turn.

The present invention employs a light beam such as the coherent andnarrow beam produced by a conventional laser, passing the beam throughthe aperture being measured and thus producing the Fraunhoferdiffraction pattern of that aperture which is then split and directed toeach of two conventional photo detectors or photocells. In onephotocell, the entire Fraunhofer pattern is received while in the otheronly the inner portion of the bright central area known as Airy Disc isreceived. By taking the ratios of the two voltages produced by the twophotodetectors, the diameter of the aperture can be simply determinedsince it is the only variable in the mathematical relation between thetwo voltages which is not known.

Moreover, the novel apparatus of this invention is particularly simple,economical and reliable. Since all constants can be determinedprecisely, the determination of aperture size is quite precise. Further,the use of a reference voltage eliminates the intensity of light as avariable so that variation in the incident light does not effect themeasurement.

Other objects and purposes of the invention will become clear afterreading the following detailed description of the drawings.

BRIEF DESCRIPTION OF THEDRAWINGS FIG. I shows the lasermicroaperturemeasurement device of this invention in use, with the laserbeam passing through an aperture which is being measured.

FIG. 2 shows a graph of the ratio of the two voltages produced by thetwo photocells versus a function in which all the variables are knownexcept the diameter a of the microaperture being measured.

DETAILED DESCRIPTION OF THE DRAWINGS Reference is now made to FIG. 1 inwhich a roughly circular microaperture 20 of diameter a and in a device28. such as a spinnerette with most of its surface removed to show theoperation of this invention, is being measured by the passage throughaperture 20 of a laser beam 22, produced by a laser 24, to the detectioncircuitry 26. The aperture 20 may be in any suitable structure or device28, and the device 28 may be rotating so that successive apertures suchas apertures 20, 23 and 25 are presented to the beam 22. Alternatively,the aperture 20 of the device 28 may be simply manually placed and heldstationary in line with the laser beam 22 during measurement. Anysuitable means for rotating the device 28 can be employed.

As mentioned above, the laser measuring instrument shown in FIG. 1 hasproved to be of particular value in measuring the diameters of theapertures of a spinnerette or jet, which is a device frequently employedin the synthetic fiber and fiberglass industries to produce fibers byextruding molten material from a rapidly rotating bowl or containerthrough a large number of apertures having very small diameters, usuallyin the range from 10 to microns. Since each of these apertures, whichnormally number from 40 to 200, must be individually checked when thedevice is manufactured to ensure that their diameters lie withinpredetermined tolerances, and since at least 3 minutes has previouslybeen required to check each individual aperture or hole, an enormousamount of time has previously been invested in determining the diametersof these holes. By properly disposing the laser 24 and circuitry 26 withregard to such a bowl or container and then spinning the bowl orcontainer so that each of the apertures is sequentially presented to thelaser beam for measurement, the diameters of each of the holes can berapidly checked in turn and in a very short time with this relativelysimple and economical measuring device. Alternatively, the bowl may beheld stationary and the laser 24 moved so as to successively passthrough each of the apertures being measured.

The laser 24 which produces the beam 22 may be of any suitable type andno particular type of laser is intended for use. Although a normalsource of incoherent light such as plain white light could possibly beemployed with this invention, a laser is preferably used because of theradiance of its light and the narrow beam which it produces.

The passage of the laser beam 22 through the aperture 20 produces adiffraction pattern in a manner which is well known. Since the aperture20 is roughly circular, the diffraction pattern produced is a Fraunhoferpattern consisting of a single bright central disc which istraditionally called Airys Disc after the English scientist G. B. Airy,and a number of concentric outlying rings. The Fraunhofer patternproduced by passing a light beam through a circular aperture isdiscussed fully in a textbook received in the Scientific Library of thePatent Office on July 21, 1964 and entitled PRINCI- PLES OF OPTICS byMax Born and Emil Wolf, and particularly on pages 392-397 therein.

As shown in FIG. 1, the laser beam 22, after passage through theaperture 20, strikes a beam splitter 30, which may be conventional,producing two split identical beams 32 and 34 which then are directed tophotodetectors 36 and 38, respectively, which are also conventional andwhich convert the intensity of the light received into an amplitude ofan electrical signal produced on an output line, the electrical signalfrom photodetector 36 being produced on line 40 and the electricalsignal from photodetector 38 being produced on line 42.

While the total amount of energy contained in the Fraunhofer patterndoes not vary directly with the size of the aperture, the distributionof that intensity with radius does vary. Further, the intensity of thepattern, and hence the energy content, at any given radius is afunction, as developed below, of the diameter of the aperture throughwhich the laser beam 22 has passed. Since the total energy is invariantwith aperture 5 diameter, the voltage amplitude produced by thephotocell 36 which admits all of the energy in the entire Fraunhoferdistribution can serve as a reference which is invariant with theaperture size. Actually since virtually all the energy is contained inthe Airy Disc and the first three or four rings, the photodetector 36need only receive the Airy Disc and the first three or four rings.

The photodetector 38 in contrast is normally designed to receive only aportion of the Fraunhofer disc and preferably, as described below,receives a circle centered at the center of the pattern and includingroughly one-half of the Airy Disc. The portion received in FIG. 1 isdesignated as area 39. The intensity distribution and hence the totalenergy within this reduced area will therefore vary with the diameter ofthe aperture, as developed below, and will thus vary the amplitude ofthe output voltage on line 42. By comparing the amplitudes of the twovoltages on lines 40 and 42 within the appropriate voltage comparatorcircuitry 48, which may be simple transistor or other circuitry ordevices, an electrical or other signal which is a function of thediameter of the aperture 20 alone can be passed to the output circuit 50which may then transform or develop the signal from the voltagecomparator 48 in any appropriate manner. Also by comparing the voltages,the determination of aperture diameter is made independent of theintensity ofthe light.

The total energy within any given area of the Fraunhofer pattern whichis within a circle with a radius at the center of the circularFraunhofer pattern can be derived as follows.

Let dE represent the energy contained between A and dA: Then.

By applying the Bessel Function Recurrence formulas,

the intensity function for Fraunhofer diffraction is written as follows:

where [(x) the intensity at positive x I,, the intensity at x o .r 2a/sa diameter of aperture s distance of aperture to the photocells 0'radius of circle from the center of the Airy Disc to a position in theFraunhofer diffraction under consideration A wavelength ofthe light beamI;'(.\')= Energy contained in a ring between 1 0 and rr=rr ,8 0/5 A Areaof photo detector opening J(x) Bessel Functions This derivation is alsoset forth in the above mentioned PRINCIPLES OF OPTICS by Born and Wolf.Thus, at any given radius, the energy in the Fraunhofer pattern within acircle with that radius can be simply determined.

Further, it will be apparent that the voltage from the referencephotodetector 36 which we shall call V, is equal to the energy Eincident upon that photocell 36 which is in fact the entire energy ofthe Fraunhofer pattern times the gain of that photocell which we shallcall g, so that:

r gr E It is further apparent that the voltage produced by thephotodetector 38 on line 42 is also a product of the energy E(8o) whichit receives within the reduced area of the pattern which it receivestimes the gain of that photocell which we shall call g,. Thus, the ratioof V, to V, can be easily shown to be g, E030) divided by g, Eand theratio is thus:

Further, since 1,, is equal to Erra /A From this equation, it can beseen that the only unknown is the letter a which is the diameter of themicroaperture. and thus the voltage ratio can be plotted against a ormore conveniently 21rafi,,/A which is in effect a multiplied by aconstant.

The above equation which represents the ratio of the voltages on lines40 and 42 is graphically represented in FIG. 2 with the voltage ratioplotted against the variable factor 2111113 ,,/A. As can be seen fromFIG. 2, if the horizontally plotted function 21raB.,/A is set byadjusting the distances shown in HQ. 1 so that the ratio V,/V,, isroughly equal to about 1:4, the relation ofthe ratio V /V to a isroughly linear and on a linear, steep portion of the curve, thusmaximizing sensitivity and linear range for variations ofa. For aSO-micron circular aperture, it has been found that the distance sshould be set to approximately l8 inches in order to obtain this ratioof about 1:4. Variations in a of plus and minus 40 percent will nowstill lie well within this linear region and permit accuratedetermination ofa.

It should be apparent from the graph of FIG. 2 that since the wavelengthof the incident light A must be held as nearly constant as possible, alaser, such as laser 24, which produces a beam having a very narrow bandwidth is particularly useful for this invention. Moreover, the accuracyof the aperture diameter determined depends on a knowledge of the exactwavelength A and the wavelength of the light produced by a laser is, ofcourse, very precisely known. Improvement in fringe contrast when laserillumination is used instead of con ventional light has already beenobserved on page 445 of a text entitled MODERN OPTICS by Earle B. Brown,received by the Scientific Library of the Patent Office on June 22,1966.

The output circuitry 50 which serves to receive the signals from thevoltage comparator 48 may be of any suitable type and may perform anyfunctions desired. For example. it may be desirable to connect theoutput circuitry 50 to a tape punch or a warning light or the like, andinclude circuitry so that a warning is given only when an aperture whosesize deviates from the standard by an amount greater than the toleranceprovided is encountered. Alternatively, it may be desirable only torecord the locations of such incorrect apertures as they are encounteredso that they can be corrected later without terminating the measurementof other apertures. In short, any suitable output circuitry whichconverts the electrical output of the voltage comparator 48 into a formwhich facilitates human or machine monitoring of the microaperturediameters can be used.

Further, the novel device of this invention is not only extremelyaccurate and reliable, it is also economical to make and use and simple.Moreover, as mentioned above, it can be used to measure aperturesindividually or it can measure a number of apertures successivelypresented to the light beam, for example, by rotating a spinnerette.Even further, the results are not dependent upon the intensity of thelight beam employed and variation of that intensity in no way effect thevalue of aperture diameter arrived at.

As mentioned above, although laser beams provide a practical andaccurate source of light, the invention can be practiced with aconventional or other like source if desired. It should also be apparentthat although the invention is especially useful for determining thediameter of microapertures, holes or apertures of any size which producesuitable diffraction patterns can be measured. Further, while in theexample above roughly circular apertures are measured, the invention canalso be used to measure dimensions in holes of any shape which produceappropriate diffraction patterns. Therefore, it should be apparent toone ofordinary skill in the art that many changes and modifications fromthe example set forth above are possible without departing from thespirit of the invention. Accordingly, the above invention is intended tobe limited only by the scope ofthe appended claims.

What is claimed is:

1. Apparatus for determining the size of an aperture comprising,

means for passing a beam of light through said aperture to produce adiffraction pattern, the distribution of energy in said pattern being afunction of the size of said aperture, first signal-producing means forreceiving said pattern with its energy distribution substantiallyundistorted and producing a signal having a characteristic which is afunction of substantially all of the energy in said pattern,

second signal-producing means for receiving a contiguous portion of saidpattern with its energy distribution undistorted and for producing asignal having a characteristic which is a function of the energy of saidpattern within a portion of said pattern less than the entire pattern,including means for preventing the remainder of said pattern from beingreceived,

means for receiving the beam of light which has passed through saidaperture and directing said diffraction pattern in that beam to saidfirst and second signal producing means with the energy distribution insaid pattern substantially undistorted, and

means for comparing the characteristics of the signals produced by thefirst and second signal-producing means to determine the size of saidaperture.

2. Apparatus as in claim 1 including a source of coherent light forproducing said beam oflight.

3. Apparatus as in claim 2 wherein said source is a laser.

4. Apparatus as in claim 1 wherein said second signalproducing meansreceives a circular area of said pattern of a given radius located atthe center of said pattern.

5. Apparatus as in claim 4 wherein said aperture is roughly circular,and said pattern is a Fraunhofer pattern comprising a central disc and aplurality of concentric rings.

6. Apparatus as in claim 5 wherein said second signalproducing meansreceives roughly half ofsaid central disc.

7. Apparatus as in claim 1 wherein said signal-producing means arephotodetectors and said signals are electrical.

8. Apparatus as in claim 4 wherein said directing means includes a beamsplitter for directing said diffraction pattern to each of saiddetectors.

9. Apparatus as in claim 8 wherein said comparing means includes meansfor obtaining the ratio of the amplitudes of the electrical signalsproduced by said photodetectors.

10. Apparatus for measuring the diameter ofa roughly circularmicroaperture comprising,

laser means for producing a beam of light and passing said beam throughsaid microaperture so that a Fraunhofer diffraction pattern, comprisinga bright central disc and a plurality of concentric bright rings andhaving an energy distribution which is a function of the size of saidmicroaperture, is produced,

a first photocell for receiving said pattern and producing a firstelectrical output signal having an amplitude which is a function ofsubstantially all of the energy within said pattern,

a second photocell for receiving a contiguous portion of said patternand producing a second electrical output signal having an amplitudewhich is a function of the energy within said contiguous portionreceived, the energy within said portion varying with the diameter ofsaid microaperture,

means for directing said pattern to said first and second photocells sothat said patterns are received by said first and second photocellssubstantially undistorted, and

means for comparing the amplitudes of said first and second signals todetermine the diameter of said microaperture.

11. A method of determining the size of an aperture comprising the stepsof,

passing a beam of light through said aperture so that a diffractionpattern is formed,

directing said diffraction pattern with its energy pattern substantiallyundistorted onto a first signal-producing means which produces a signalhaving a characteristic which is a function of substantially all of theenergy of said diffraction pattern,

directing said diffraction pattern onto a second signalproducing meansso that said second signal-producing means receives only a contiguousportion of said pattern, the energy portion of said pattern beingsubstantially undistorted, and produces a signal having a characteristicwhich is a function of the energy within said portion of saiddiffraction pattern, the energy within said portion being a functionofthe size of said aperture, and

comparing said characteristics of said first and second signals todetermine the size of said aperture.

12. A method of determining the size of a roughly circular microaperturecomprising the steps of,

producing a laser beam,

directing said laser beam through said aperture,

splitting said diffraction pattern to produce two patterns with theenergy distribution substantially undistorted,

directing the first of said two patterns onto a first photodetectorwhich produces an electrical signalhaving an amplitude which is afunction of substantially all of the energy within said diffractionpattern,

directing a contiguous portion of the second of said two patterns onto asecond photodetector which produces an electrical signal having anamplitude which is a function of the energy within said portion of saiddiffraction pattern, the energy within said portion being a function ofthe size of said aperture, and

comparing the amplitudes of said electrical signals to determine thediameter of said aperture.

13. A method of measuring the diameters of a number of microapertures ina device comprising the steps of,

producing a laser beam,

presenting each of said number of microapertures successively so thatsaid beam passes through each said aperture sequentially, thus producinga Fraunhofer diffraction pattern for each microaperture comprising abright central disc and a plurality of concentric rings and having anenergy distribution which is a function of the diameter of themicroaperture through which the beam is passing,

center of said portion being a function of the cube of the diameter ofthe microaperturc through which the beam is passing, and comparing theamplitudes of the first and second signals produced for eachmicroaperture through which said beam passes to measure the diameters ofall of said number of microapertures. 14. A method as in claim 13wherein said presenting includes the step of rotating said device.

1. Apparatus for determining the size of an aperture comprising, meansfor passing a beam of light through said aperture to produce adiffraction pattern, the distribution of energy in said pattern being afunction of the size of said aperture, first signal-producing means forreceiving said pattern with its energy distribution substantiallyundistorted and producing a signal having a characteristic which is afunction of substantially all of the energy in said pattern, secondsignal-producing means for receiving a contiguous portion of saidpattern with its energy distribution undistorted and for producing asignal having a characteristic which is a function of the energy of saidpattern within a portion of said pattern less than the entire pattern,including means for preventing the remainder of said pattern from beingreceived, means for receiving the beam of light which has passed throughsaid aperture and directing said diffraction pattern in that beam tosaid first and second signal producing means with the energydistribution in said pattern substantially undistorted, and means forcomparing the characteristics of the signals produced by the first andsecond signal-producing means to determine the size of said aperture. 2.Apparatus as in claim 1 including a source of coherent light forproducing said beam of light.
 3. Apparatus as in claim 2 wherein saidsource is a laser.
 4. Apparatus as in claim 1 wherein said secondsignal-producing means receives a circular area of said pattern of agiven radius located at the center of said pattern.
 5. Apparatus as inclaim 4 wherein said aperture is roughly circular, and said pattern is aFraunhofer pattern comprising a central disc and a plurality ofconcentric rings.
 6. Apparatus as in claim 5 wherein said secondsignal-producing means receives roughly half of said cenTral disc. 7.Apparatus as in claim 1 wherein said signal-producing means arephotodetectors and said signals are electrical.
 8. Apparatus as in claim4 wherein said directing means includes a beam splitter for directingsaid diffraction pattern to each of said detectors.
 9. Apparatus as inclaim 8 wherein said comparing means includes means for obtaining theratio of the amplitudes of the electrical signals produced by saidphotodetectors.
 10. Apparatus for measuring the diameter of a roughlycircular microaperture comprising, laser means for producing a beam oflight and passing said beam through said microaperture so that aFraunhofer diffraction pattern, comprising a bright central disc and aplurality of concentric bright rings and having an energy distributionwhich is a function of the size of said microaperture, is produced, afirst photocell for receiving said pattern and producing a firstelectrical output signal having an amplitude which is a function ofsubstantially all of the energy within said pattern, a second photocellfor receiving a contiguous portion of said pattern and producing asecond electrical output signal having an amplitude which is a functionof the energy within said contiguous portion received, the energy withinsaid portion varying with the diameter of said microaperture, means fordirecting said pattern to said first and second photocells so that saidpatterns are received by said first and second photocells substantiallyundistorted, and means for comparing the amplitudes of said first andsecond signals to determine the diameter of said microaperture.
 11. Amethod of determining the size of an aperture comprising the steps of,passing a beam of light through said aperture so that a diffractionpattern is formed, directing said diffraction pattern with its energypattern substantially undistorted onto a first signal-producing meanswhich produces a signal having a characteristic which is a function ofsubstantially all of the energy of said diffraction pattern, directingsaid diffraction pattern onto a second signal-producing means so thatsaid second signal-producing means receives only a contiguous portion ofsaid pattern, the energy portion of said pattern being substantiallyundistorted, and produces a signal having a characteristic which is afunction of the energy within said portion of said diffraction pattern,the energy within said portion being a function of the size of saidaperture, and comparing said characteristics of said first and secondsignals to determine the size of said aperture.
 12. A method ofdetermining the size of a roughly circular microaperture comprising thesteps of, producing a laser beam, directing said laser beam through saidaperture, splitting said diffraction pattern to produce two patternswith the energy distribution substantially undistorted, directing thefirst of said two patterns onto a first photodetector which produces anelectrical signal having an amplitude which is a function ofsubstantially all of the energy within said diffraction pattern,directing a contiguous portion of the second of said two patterns onto asecond photodetector which produces an electrical signal having anamplitude which is a function of the energy within said portion of saiddiffraction pattern, the energy within said portion being a function ofthe size of said aperture, and comparing the amplitudes of saidelectrical signals to determine the diameter of said aperture.
 13. Amethod of measuring the diameters of a number of microapertures in adevice comprising the steps of, producing a laser beam, presenting eachof said number of microapertures successively so that said beam passesthrough each said aperture sequentially, thus producing a Fraunhoferdiffraction pattern for each microaperture comprising a bright centraldisc and a plurality of concentric rings and having an energydistribution which is A function of the diameter of the microaperturethrough which the beam is passing, directing each said pattern with itsenergy pattern substantially undistorted onto a first photodetectorwhich produces an electrical signal having an amplitude which is afunction of substantially all the energy within said pattern, the energywithin said portion being a function of the square of the diameter ofthe microaperture through which the beam is passing, and onto a secondphotodetector which produces a second electrical signal having anamplitude which is a function of the energy within a circular contiguousarea of said pattern centered at the center of said portion being afunction of the cube of the diameter of the microaperture through whichthe beam is passing, and comparing the amplitudes of the first andsecond signals produced for each microaperture through which said beampasses to measure the diameters of all of said number of microapertures.14. A method as in claim 13 wherein said presenting includes the step ofrotating said device.